Various central air conditioning systems are known in the art which provide conditioned air to a plurality of rooms wherein the temperature of each of the rooms may be controlled independently.
One such prior art attempt is the VVT System marketed by the Carrier Corporation of Minnville, Tenn. in the United States and described in the Carrier publication No. 13301 of Aug. 20, 1990. The VVT System divides the rooms to be air conditioned into a number of zones and controls the amount of conditioned air entering each zone by means of controllable dampers operating in response to the temperature in each of the zones.
As the temperature in each of the rooms changes, the dampers are opened or closed accordingly, thereby maintaining a desired temperature in each of the rooms or zones. If several of the rooms or zones are at the desired temperature, the dampers to these rooms are closed, thereby preventing the flow of conditioned air into these rooms. The dampers leading to the rooms which are not at the desired temperature are opened and the flow of conditioned air is directed to those rooms only. As long as there is a requirement for conditioned air from any of the rooms, the compressor of the heat pump must operate, thereby consuming substantial amounts of electrical energy. The excess thermal capacity of the compressor when operating under such part load conditions is wasted
Another such prior art device is described in U.S. Pat. No. 4,635,455 to Otsuka et al. The Otsuka prior art device measures the heat load in each room by means of temperature sensors and controls the quantity of air to be directed to each room by means of variable dampers. The pressure in the main air duct is also measured and the speed of the blower adjusted accordingly. Under part load conditions, the speed of the compressor is reduced so as to conserve energy.
As in know in the art, operation of a compressor at reduced speeds results in inefficient operation of the compressor as well as requiring an expensive speed controller for controlling the speed of the electric motor which drives the compressor. Furthermore, a compressor capable of operating at a number of different speeds or over a continuous speed range is more costly than a compressor designed to operate at a single speed.